There is an ongoing effort to develop systems that are more energy-efficient. A large proportion (some estimates are as high as twenty-five percent) of the electricity generated in the United States each year goes to lighting, a large portion of which is general illumination (e.g., downlights, flood lights, spotlights and other general residential or commercial illumination products). Accordingly, there is an ongoing need to provide lighting (of all types, including general illumination) that is more energy-efficient, without sacrificing other desired properties of lighting (or limiting detraction of other desired properties).
Solid state light emitters (e.g., light emitting diodes) are receiving much attention due to their energy efficiency. It is well known that incandescent light bulbs are very energy-inefficient light sources—about ninety percent of the electricity they consume is released as heat rather than light. Fluorescent light bulbs are more efficient than incandescent light bulbs (by a factor of about 10) but are still less efficient than solid state light emitters, such as light emitting diodes.
In addition, as compared to the normal lifetimes of solid state light emitters, e.g., light emitting diodes, incandescent light bulbs have relatively short lifetimes, i.e., typically about 750-1000 hours. In comparison, light emitting diodes, for example, have typical lifetimes between 50,000 and 70,000 hours. Fluorescent bulbs have longer lifetimes (e.g., 10,000-20,000 hours) than incandescent lights, but provide less favorable color reproduction. The typical lifetime of conventional fixtures is about 20 years, corresponding to a light-producing device usage of at least about 44,000 hours (based on usage of 6 hours per day for 20 years). Where the light-producing device lifetime of the light emitter is less than the lifetime of the fixture, the need for periodic change-outs is presented. The impact of the need to replace light emitters is particularly pronounced where access is difficult (e.g., vaulted ceilings, bridges, high buildings, highway tunnels) and/or where change-out costs are extremely high.
General illumination devices are typically rated in terms of their color reproduction. Color reproduction is typically measured using the Color Rendering Index (CRI Ra). CRI Ra is a modified average of the relative measurements of how the color rendition of an illumination system compares to that of a reference radiator when illuminating eight reference colors, i.e., it is a relative measure of the shift in surface color of an object when lit by a particular lamp. The CRI Ra equals 100 if the color coordinates of a set of test colors being illuminated by the illumination system are the same as the coordinates of the same test colors being irradiated by the reference radiator.
Daylight has a high CRI (Ra of approximately 100), with incandescent bulbs also being relatively close (Ra greater than 95), and fluorescent lighting being less accurate (typical Ra of 70-80). Certain types of specialized lighting have very low CRI (e.g., mercury vapor or sodium lamps have Ra as low as about 40 or even lower). Sodium lights are used, e.g., to light highways—driver response time, however, significantly decreases with lower CRI Ra values (for any given brightness, legibility decreases with lower CRI Ra).
The color of visible light output by a light emitter, and/or the color of blended visible light output by a plurality of light emitters can be represented on either the 1931 CIE (Commission International de I'Eclairage) Chromaticity Diagram or the 1976 CIE Chromaticity Diagram. Persons of skill in the art are familiar with these diagrams, and these diagrams are readily available (e.g., by searching “CIE Chromaticity Diagram” on the internet).
The CIE Chromaticity Diagrams map out the human color perception in terms of two CIE parameters x and y (in the case of the 1931 diagram) or u′ and v′ (in the case of the 1976 diagram). Each point (i.e., each “color point”) on the respective Diagrams corresponds to a particular color. For a technical description of CIE chromaticity diagrams, see, for example, “Encyclopedia of Physical Science and Technology”, vol. 7, 230-231 (Robert A Meyers ed., 1987). The spectral colors are distributed around the boundary of the outlined space, which includes all of the hues perceived by the human eye. The boundary represents maximum saturation for the spectral colors.
The 1931 CIE Chromaticity Diagram can be used to define colors as weighted sums of different hues. The 1976 CIE Chromaticity Diagram is similar to the 1931 Diagram, except that similar distances on the 1976 Diagram represent similar perceived differences in color.
In the 1931 Diagram, deviation from a point on the Diagram (i.e., “color point”) can be expressed either in terms of the x, y coordinates or, alternatively, in order to give an indication as to the extent of the perceived difference in color, in terms of MacAdam ellipses. For example, a locus of points defined as being ten MacAdam ellipses from a specified hue defined by a particular set of coordinates on the 1931 Diagram consists of hues that would each be perceived as differing from the specified hue to a common extent (and likewise for loci of points defined as being spaced from a particular hue by other quantities of MacAdam ellipses).
Since similar distances on the 1976 Diagram represent similar perceived differences in color, deviation from a point on the 1976 Diagram can be expressed in terms of the coordinates, u′ and v′, e.g., distance from the point=(Δu′2+Δv′2)1/2. This formula gives a value, in the scale of the u′ v′ coordinates, corresponding to the distance between points. The hues defined by a locus of points that are each a common distance from a specified color point consist of hues that would each be perceived as differing from the specified hue to a common extent.
A series of points that is commonly represented on the CIE Diagrams is referred to as the blackbody locus. The chromaticity coordinates (i.e., color points) that lie along the blackbody locus obey Planck's equation: E(λ)=Aλ−5/(eB/T)−1), where E is the emission intensity, λ is the emission wavelength, T is the color temperature of the blackbody and A and B are constants. The 1976 CIF, Diagram includes temperature listings along the blackbody locus. These temperature listings show the color path of a blackbody radiator that is caused to increase to such temperatures. As a heated object becomes incandescent, it first glows reddish, then yellowish, then white, and finally blueish. This occurs because the wavelength associated with the peak radiation of the blackbody radiator becomes progressively shorter with increased temperature, consistent with the Wien Displacement Law. Illuminants that produce light that is on or near the blackbody locus can thus be described in terms of their color temperature.
The most common type of general illumination is white light (or near white light), i.e., light that is close to the blackbody locus, e.g., within about 10 MacAdam ellipses of the blackbody locus on a 1931 CIE Chromaticity Diagram. Light with such proximity to the blackbody locus is referred to as “white” light in terms of its illumination, even though some light that is within 10 MacAdam ellipses of the blackbody locus is tinted to some degree, e.g., light from incandescent bulbs is called “white” even though it sometimes has a golden or reddish tint; also, if the light having a correlated color temperature of 1500 K or less is excluded, the very red light along the blackbody locus is excluded.
The emission spectrum of any particular light emitting diode is typically concentrated around a single wavelength (as dictated by the light emitting diode's composition and structure), which is desirable for some applications, but not desirable for others, (e.g., for providing general illumination, such an emission spectrum provides a very low CRI Ra).
Because light that is perceived as white is necessarily a blend of light of two or more colors (or wavelengths), no single light emitting diode junction has been developed that can produce white light.
“White” solid state light emitting lamps have been produced by providing devices that mix different colors of light, e.g., by using light emitting diodes that emit light of differing respective colors and/or by converting some or all of the light emitted from the light emitting diodes using luminescent material. For example, as is well known, some lamps (referred to as “RGB lamps”) use red, green and blue light emitting diodes, and other lamps use (1) one or more light emitting diodes that generate blue light and (2) luminescent material (e.g., one or more phosphor materials) that emits yellow light in response to excitation by light emitted by the light emitting diode, whereby the blue light and the yellow light, when mixed, produce light that is perceived as white light. While there is a need for more efficient white lighting, there is in general a need for more efficient lighting in all hues.
There are many situations where it is desired for the intensity of light exiting a lighting device to be highly uniform. For instance, with general illumination devices, it is often desired to have a lighting device where the variance of intensity between different locations from which light exits the lighting device (e.g., the lens) is limited or minimized. The challenges presented by a desire for uniformity can be increased where the size of the region from which light is emitted is increased, e.g., in the case of troffer lights, which are often designed to fit in square spaces in ceilings measuring roughly two feet by two feet or in rectangular spaces in ceilings measuring roughly two feet by four feet.
There are many situations where it is desired for the light exiting a lighting device to create less glare. Again, such challenges can be more pronounced with lights having larger areas of light emission.
For the above and other reasons, there is an ongoing need for a lighting device that provides good efficiency, good color uniformity of emitted light, good intensity uniformity of emitted light, and/or low glare. In some cases, it is further desired that the light emitted from the lighting device be white and have acceptable color temperature and/or high CRI Ra, and/or that the lighting device have a long useful life.